The present invention relates to electrostatic lens systems and particularly to a system for accurately focusing a charged particle beam to a very small spot.
Apparatus for generating charged particle microbeams for use in laboratory instruments such as scanning electron microscopes and the like require highly accurate beam focusing. The electrostatic focusing lens system employed should be of high precision, exhibiting a high degree of concentricity and circularity, to generate a beam that can be concentrated to a very small spot on a target or specimen.
The lens electrodes in such a focusing lens system need to be provided with precisely aligned apertures through which the charged particle beam passes, while the electrodes must also be spaced at precise distances in parallel relation to one another. Stacked ceramic cylinders have been employed for spacing and supporting focusing electrodes, wherein the cylinders provide an outer reference. An accurately manufactured cylinder peripherally supports an electrode or electrodes therewithin such that the electrodes are then centered. However, not only must the ceramic cylinders be machined to high tolerances, but also the lens electrodes must have accurately machined circumferential surfaces where they are received by the cylinders. Since the tolerances at the interfaces are added together, it can be very difficult to hold accurate concentricity and parallelism.
More commonly, a glassed structure is employed wherein focusing electrodes are supported on metal wires that extend inwardly from glass members. This method may employ an aluminum centering rod or a mandrel that is later removed chemically after the glassed supporting structure has been formed. However, focusing columns manufactured in this way tend to vibrate and in particular resonate at certain frequencies. The result is undesired beam vibration. Furthermore, these structures are difficult to manufacture since considerable manufacturing skill is required.